Metal-clad insulated electrical heater

ABSTRACT

A low mass insulated heater that can be bonded with low thermal drop to an object to be heated consists of a planar arrangement of a heater wire which, after being covered with a thin coating of insulation, is coated with material that is sintered or alloyed to form a metallic sheath. The sheath is brazed directly to a member to be heated to form an integrally bonded structure in which the composite materials all have similar thermal expansion characteristics.

United States Patent [72] Inventor James E. Beggs Schenectady, N.Y. [21] Appl. No. 811,067 [22] Filed Mar. 27, 1969 [45 Patented May 25, 1971 [73] Assignee General Electric Company [54] METAL-GLAD INSULATED ELECTRICAL HEATER 8 Claims, 4 Drawing Figs.

[52] 11.8. C1 313/340, 219/462, 219/530, 313/270, 313/341 [51] Int. Cl H01j 1/24 [50] Field otSearch..... 219/458, 7, r 530, 540; 313/340, 341, 271, 337, 270 29/611 [56] References Cited UNITED STATES PATENTS 1,975,870 10/1934 Shrader 313/340 2,075,910 4/1937 Robinson 313/340 2,131,909 10/1938 Umbreit 313/340 2,504,335 4/1950 Jonker 313/340 3,117,249 1/1964 Winters 313/337 3,221,203 11/1965 Ragland, Jr... 313/271 3,400,294 9/1968 Kling 313/340 Primary ExaminerVolodymyr Y. Mayewsky Att0rneysPaul A. Frank, John F. Ahern, Julius J.

Zaskalicky, Joseph B. Forman, Frank L. Neuhauser and Oscar B. Waddell METAL-CLAD INSULATED ELECTRICAL HEATER The present invention relates to electrical heaters and, in particular, to a low mass insulated heater that can be bonded with low thermal drop to an object to be heated and a method of manufacturing such heater.

In electrical heating systems, many expedients are used to minimize the thermal drop between the heater wire and the object to be heated while still providing sufficient electrical insulation. One electrical structure in which the problem of reducing thermal drop between the heater and the object to be heated exists is the cathode for an electric discharge device and, particularly, a low mass cathode capable of reaching a high operating temperature in an extremely short period of time. One cathode structure of a type presently used is shown in U.S. Pat. No. 3,400,294-Kling, granted Sept. 3, 1968 and assigned to the assignee of the present invention. In the structure of this patent, the heater wire is embedded in a ceramic refractory material which is in bonded relation with the metal disc of the cathode. A metal powder is sintered or deposited on the underside of the cathode to provide interlocking arrangement with the refractory material. While such structures show marked improvement over prior ones in their rapidity of heating and resistance to mechanical shocks, because of the various expansions of the materials used in assembly and in the procedure used for bonding, it is difficult to avoid warping of the cathode lid, in many instances. In microwave electron discharge devices, the cathode is spaced very close to associated grid electrodes and any warping of the cathode structure impairs the operation of the device.

Another instance in which it would be desirable to provide uniform electrical heating of a member with low thermal drop is in electrical cooking equipment and other domestic heaters.

While the familiar Calrod electrical heating unit has given excellent service in such equipment, certain improvements could be made if there were available a flat, continuous, tabletop-type heating surface which can be brought to a desired temperature rapidly with little thermal drop between the surface and the heater wire and without warping the flat surface.

It is an object of my present invention to provide a new and improved insulated heater and methods of manufacturing such heater, in which the heater and the object being heated have similar thermal expansion properties so that stresses are not introduced.

It is still another object of my invention to provide a low mass insulated heater which can be bonded with low thermal drop to the object to be heated and in which the materials of the heater and the objects to he heated'have closely similar thermal expansion characteristics.

In its broadest aspects, my invention consists in constructing an insulated electrical heater by forming coplanar turns of a heater wire, covering the wire with a thin, tightly adhering coating of an insulating material, depositing a metal overcoating on the insulated material, and applying a thin layer of brazing or alloying material to at least one surface of the overcoated coplanar turns which then is positioned adjacent the object to be heated and heated until a tight, impervious bond is formed between the object and the composite heater, the various materials of the heater structure and the object being chosen to have closely similar thermal expansion characteristics.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of the specification.

The invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which FIG. 1 is a perspective view illustrating the method of manufacturing the metal clad insulated heater;

FIG. 2 is a side view of the heater of FIG. 1 showing a thin layer of additional metal applied to one surface of the heater;

FIG. 3 illustrates another configuration of the heater of my invention; and

FIG. 4 illustrates a cathode for an electron discharge device embodying the heater of my invention.

In practicing the process whose steps are illustrated in FIG. 1, a heater wire 1 is formed into a layer of coplanar turns 2 of the wire. While any desired configuration of the wire may be used, in this figure the layer is shown as comprising a plurality of spiral turns 2 of wire 1. After the wire is formed into the desired configuration, it is covered with a coating of insulating material 3. The coating may be applied by any conventional techniques, such as flame spraying, chemical vapor deposition, or cataphoresis. The chemical vapor deposition technique is particularly suited since it provides a very thin impervious coating on the wire, the coating having a high dielectric strength. After the insulation has been applied to the wire, the insulated wire is covered with an overcoating 4 of metal. Overcoat 4 may likewise be applied to the insulated wire by flame spraying, schoop spraying, or any other suitable deposition process. Alternatively, the material is applied in the form of a metal oxide and later converted to the pure metal by heating in a reducing-atmosphere, such as dry hydrogen. The next step in the process is to coat one surface of the layer of wire with a metal powder 5 and braze it to or alloy it with the object or surface 6 which is to be heated. As shown in FIG. 2, the object to be heated comprises a flat metal plate 6. After overcoated turns 4 and powder 5 are placed on plate 6, the resultant structure is heated so that the heater layer 2 and powder 5 are brazed or alloyed to plate 6. Of course, layer 5 may be attached to the composite heater prior to its attachment to plate 6.

In constructing my heater, the wire 1 may comprise any suitable heater wire, for example, tungsten, nickel, or molybdenum. l have found molybdenum particularly suitable for constructing a cathode structure for an electron discharge device since it is ductile and can be easily shaped into a layer of coplanar turns. The material of insulating coating 3 is selected so that its thermal expansion characteristics match that of the heater wire 1. Suitable insulating materials are alumina, a mixture of boron nitride and alumina, a mixture of magnesium oxide and boron nitride, and a mixture of magnesium oxide and alumina. In particular, a mixture of alumina and boron nitride powder provides an insulating material having thermal expansion characteristics which matches that of molybdenum. The selected material is deposited on the formed wire by flame spraying, chemical vapor deposition, or cataphoresis.

The material of overcoating 4 is likewise selected so that its thermal expansion characteristic matches that of both wire 1 and insulating coating 3. Metals found suitable for forming such overcoating are molybdenum, tungsten, and nickel. The overcoating may likewise be applied by any suitable method such as flame spraying, chemical vapor deposition, or cataphoresis. Again, tungsten or molybdenum oxides may be used for overcoating 4 then converted to the pure metal by heating in a reducing atmosphere. Powder 5 may comprise molybdenum, nickel, or a mixture of molybdenum and nickel powders depending on the metals of overcoat 4 and plate 6 and the brazing or alloying temperature is selected accordingly. For example, an alloy of molybdenum and nickel will permit brazing a heater structure having an overcoating 4 of tungsten, molybdenum, or nickel to a tungsten, molybdenum, or nickel disc or plate 6 as the case may be, by hydrogen brazing at l,350 C.

FIG. 3 shows a planar configuration in which the layer of turns of heater wire is not spiral but permits other configurations. The configuration shown is particularly suitable for use in domestic heating appliances, having a continuous surface or panel to which the heater is brazed or otherwise bonded, suitable terminals (not shown) being attached to the uncoated ends of wire 1.

FIG. 4 shows the heater of my invention used in conjunction with a cathode for an electron discharge device. Cathode 8 comprises a tungsten disc having transverse slots 13 and is formed in the manner shown and described in my U.S. Pat. No. 3,334,263, granted Aug. 1, i967, and assigned to the assignee of this present invention. The structure of FIG. 4 also employs a backup disc 9 which may be desirable, in some instances, to maintain the cathode flat and further assist in preventing warping. The use of a backup disc 9 is particularly desirable in those instances where the heater and cathode structures are not made entirely of materials having similar thermal expansion characteristics. For example, if cathode 8 is of tungsten while heater wire 1 consists of molybdenum with an insulating material 3 of alumina and an overcoating of molybdenum, then it is desirable to use a tungsten backup disc 9.

Disc 9 may be formed of the same metal as disc 8 and have openings 10 for receiving the terminal portions of the heater. lnterposed between one surface of disc 9 and the turns 2 of my heater is a metal layer 11 suitable for brazing to or alloying with disc 9 and the overcoating material 4 on the heater. Layer 11 may comprise, for example, a mixture of nickel and molybdenum powders which is coated on the overcoating metal 4 at the same time that metal layer 5 is placed between the other surface of the heater and cathode 8. The entire assembly is then hydrogen brazed at a suitable temperature, for

- example 1,350" C. to provide an integral unit. In a typical structure, the disc 9 may be, for example, a very thin tungsten disc having a thickness of the order of 0.010 inch. A tungsten cathode 8 employing my bonded heater construction has been found to remain flat after many repeated heating and cooling cycles. Such a structure is particularly desirable for microwave electron tubes in which very close spacing is desired between the cathode structure and any grid of the discharge device. In a typical cathode structure formed in this manner, the cathode disc 8 comprises a porous tungsten member. Wire 1 is molybdenum wire. Insulating coating 3 is a mixture of boron nitride and alumina and overcoating 4 is a layer of molybdenum which is applied by flame spraying. The brazed metal layers 5 and 11 comprise a molybdenum nickel alloy having a melting temperature of approximately 1,3 l 5 C. Backup disc 9 comprises a l0-mil tungsten disc.

In forming heaters according to my invention, backup disc 9 is needed only if the heater and the surface 6 to which it is brazed do not I have perfectly matched thermal expansion characteristics. The use of a backup disc 9, in such instances, assists to maintain the structure and plate 6 perfectly flat despite cycling over a broad temperature range.

While numerous combinations of materials may be employed in constructing my improved metal clad insulated heater, the following chart lists a number of combinations of materials having similar or compatible thermal expansion characteristics.

ment of a mass of insulating material on the object to be heated and permits more rapid heating without warping of the object being heated. In operating such a structure, the heater can be repeatedly cycled to red-hot temperatures without causing separation of the heater from the object being heated.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broadest aspects and I, therefore, intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention. V V p U A 1 What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical heater comprising coplanar turns of a heater wire, input terminals for said wire, the metal of the wire being selected from the group consisting of molybdenum and tungsten; a coating of insulating material surrounding said wire, said insulating material being selected from a group consisting of alumina, a mixture of boron nitride and alumina, and a mixture of boron nitride and magnesium oxide; a metal overcoating surrounding the insulating material, the metal of said overcoating being selected from the group consisting of molybdenum and tungsten; a layer of metal attached to at least one side of said overcoated wire, the metal of said layer consisting of molybdenum or an alloy of molybdenum and nickel; the proportions of said insulating materials and the materials of said wire, overcoating metal, and metal layer all being selected to have closely matched thermal expansion characteristics over a wide range of temperatures; and a metal object to be heated having a flat surface parallel with and integrally attached to said metal layer, said metal object having similar matching thermal expansion characteristics.

2. The electrical heater of claim 1 in which said metal object comprises a cathode disc attached to said metal layer.

3. The electrical heater of claim 2 in which a layer of metal is attached to both sides of said overcoated grid and a backup disc is attached to the metal layer on the side opposite the cathode disc.

4. The heater of claim 1 in which the heater wire and overcoating consist of molybdenum, the metal layer is an alloy of molybdenum and nickel, and the insulating material is alumina.

5. The heater of claim 1 in which the heater wire and overcoating consist of tungsten, the metal layer is an alloy of molybdenum and nickel, and the insulating material is boron nitride.

6. The heater of claim 1 in which the metal of said object is selected from the group consisting of molybdenum, tungsten and nickel.

Heated object Wire (1) Insulation (3) Overcoat (4) Braze metal (5) (11) (6) (8) Molybdenum Alumina Molybdenum.. Molybdenum-niekel Molybdenum.

Do Alumina plus boron nitride d0 d0 Do. Tungsten... Boron nitride Tungsten do Tungsten Do Alumina plus boron nitride do D Molybdenum Alumina Molybdenum D0. Nickel Alumina plus magnesia- Nickel .do Nickel.

l Tungsten backup plate (9) desirable.

7. The heater of claim 6 in which the metal of the object to be heated is tungsten.

8. The heater of claim 6 in which the metal of the object to be heated is molybdenum. 

1. An electrical heater comprising coplanar turns of a heater wire, input terminals for said wire, the metal of the wire being selected from the group consisting of molybdenum and tungsten; a coating of insulating material surrounding said wire, said insulating material being selected from a group consisting of alumina, a mixture of boron nitride and alumina, and a mixture of boron nitride and magnesium oxide; a metal overcoating surrounding the insulating material, the metal of said overcoating being selected from the group consisting of molybdenum and tungsten; a layer of metal attached to at least one side of said overcoated wire, the metal of said layer consisting of molybdenum or an alloy of molybdenum and nickel; the proportions of said insulating materials and the materials of said wire, overcoating metal, and metal layer all being selected to have closely matched thermal expansion characteristics over a wide range of temperatures; and a metal object to be heated having a flat surface parallel with and integrally attached to said metal layer, said metal object having similar matching thermal expansion characteristics.
 2. The electrical heater of claim 1 in which said metal object comprises a cathode disc attached to said metal layer.
 3. The electrical heater of claim 2 in which a layer of metal is attached to both sides of said overcoated grid and a backup disc is attached to the metal layer on the side opposite the cathode disc.
 4. The heater of claim 1 in which the heater wire and overcoating consist of molybdenum, the metal layer is an alloy of molybdenum and nickel, and the insulating material is alumina.
 5. The heater of claim 1 in which the heater wire and overcoating consist of tungsten, the metal layer is an alloy of molybdenum and nickel, and the insulating material is boron nitride.
 6. The heater of claim 1 in which the metal of said object is selected from the group consisting of molybdenum, tungsten and nickel.
 7. The heater of claim 6 in which the metal of the object to be heated is tungsten.
 8. The heater of claim 6 in which the metal of the object to be heated is molybdenum. 